This invention relates to inductively coupled data links. More particularly, the invention establishes combined power transfer and reliable “forward telemetry” data communications between an external transmitter and an internal miniature low power receiver such as might be found in an electronic medical implant, The invention described herein was developed to communicate with an implantable medical device, but may also be effectively applied to other applications, such as some RFID transponders, for which data must be sent from an external transmitter to internal device electronics. In all of these devices miniaturization is important. Therefore, optimizing the combination of power and communication transfer from a transmitter to implant or miniature internal device while maintaining small size is crucial. To accomplish this an efficient means of remotely powering and providing telemetry data to the implant, as well as a robust means of demodulating the telemetered data within the implant are needed. A novel transmitter embodiment, and a novel receiver embodiment is described. The transmitter and receiver, in combination, yield an improved forward data telemetry system.
Short-range electronic communication using magnetic coupling is presently used in many diverse applications. These applications include implantable medical electronics, radio frequency identification systems, wireless control of actuators, and sensor-to-indicator communications.
In a typical magnetically-coupled system, an external transmitter generates a relatively strong AC magnetic field by driving radiofrequency (RF) current into a transmitter coil. The external transmitter is typically comprised of a High-Q resonant circuit, commonly known in the art to be made up of low-loss RLC components, having a Q greater than about twenty. The transmitter modulates this field, in accordance with an incoming stream of data, by changing the nature of the transmitter coil current. An internal coil, connected to the internal receiver, changes the magnetic field into a current. The internal receiver detects data modulation on this current and extracts the data, which then may be used by other functional elements in the internal assembly. The magnetic field may also be used to power the internal electronics, thereby removing the requirement for a battery in the internal electronics. The combination of the external transmitter coil and the internal coil, contained within the internal device, is called a magnetic link. A device powered solely by the external magnetic field, through the magnetic link, is termed “passive”.
Various electrostatic and electromagnetic transponders (described in US Patents issued to Vega et al. U.S. Pat. No. 6,147,605, Carrender et al. U.S. Pat. No. 6,107,917, O'Toole et al U.S. Pat. No. 6,130,602 and U.S. Pat. No. 6,157,230) are described for use as RFID tags. These transponders typically operate in the medium to high frequency range (8 MHz-1 GHz) and can have long range and demonstrate high data bandwidths. Magnetically coupled systems typically operate at lower frequencies (10 kHz-10 MHz) and typically demonstrate shorter range and lower data bandwidths. A magnetically coupled system however is minimally affected by immersion in a conductive medium and orientation of the receiver coil in the exciting magnetic field. Magnetic coupling also enables the use of passive transponders. These are required features for an implantable transponder. The invention described herein is a magnetically coupled data and power transmission system, with modulation and demodulation techniques developed and applied in a manner to optimize the performance of the system. To fully understand and appreciate the improvements of the instant invention, the prior art must be explored while considering the system-level objectives of the invention.
For optimal system performance, a transmitter in this type of application must generate a strong magnetic field as efficiently as possible. Low power conversion efficiency results in weaker magnetic fields, the unnecessary generation of heat, and shorter battery life, for portable applications. In order to have a reasonable separation between the transmitter and the internal device, large currents must exist in the transmitter coil, and transmitter coil quality factor (Q) is the first and necessary limitation of transmitter power efficiency. Often, a resonant capacitor is used with the inductance of the transmitter coil, and the non-infinite Q of this capacitor further degrades the efficiency of the transmitter. Transmitter embodiments exist using topologies that require that a substantial portion of the transmitter coil current pass through semiconductor switching elements (see, for example the US patents issued to Silvian U.S. Pat. No. 4,681,111, Biegel U.S. Pat. No. 5,559,507, Weijand et al. U.S. Pat. No. 5,999,857, Crosby et al. U.S. Pat. No. 4,532,930, and Milheiser U.S. Pat. No. 5,166,676). However, in these applications, the semiconductor losses are usually larger than the Q losses of the resonant elements, and substantially degrade the system's power efficiency.
In order to communicate with the internal device over the magnetic link, the current in the transmitter coil must be modulated. The transmitter modulation method must have a minimal impact on transmitter power efficiency, and must be readily implemented with available components. If powering of the internal device is required, the modulation method should not result in the disappearance of the powering magnetic field for a significant period of time, since that would interrupt power to the internal electronics. The instant invention uses frequency-shift-keyed (FSK) modulation to transfer data from the transmitter to the receiver while maintaining a magnetic field of near constant amplitude in order to supply continuous receiver power.
Weijand et al, '857. describe an FSK transmitter topology where the resonant frequency of a series-resonant circuit is selectively altered by using a MOSFET switch to place a capacitor directly across the antenna coil. In addition to the previously mentioned losses resulting from series switching of the transmitter coil current, this topology is limited in its application to low transmitter power. To achieve coil currents of several Amperes in the 100 kHz, or above, frequency range, it is not unusual to observe transmitter coil voltages approaching 1000 Volts. The MOSFET switching element as used by Weijand must withstand this voltage, necessitating the use of a high-voltage power MOSFET. To maintain acceptable power efficiency when the MOSFET is conducting, the “On” resistance of the MOSFET device must be as low as possible, necessitating a physically large MOSFET device. Such a device will demonstrate large parasitic drain and gate capacitances. The large gate capacitance degrades power efficiency because of the large drive currents required to initiate MOSFET switching. The large drain capacitance appears in parallel with the transmitter coil when the MOSFET is off, thereby lowering the resonant frequency of the network when the cited FSK system is in the high-frequency state. This drain capacitance will therefore limit the frequency of operation of the system. In practice implementing the techniques described in the '857 patent requires the use of a high-voltage, low-on resistance, low-capacitance MOSFET, and such MOSFET devices are not presently available. It is an object of this invention to use a transmitter topology that enables the use of a low-voltage MOSFET, low-on-resistance MOSFET, high-capacitance MOSFET, to alter the operating frequency of a high-power, High-Q resonant transmitter circuit.
Griffith, U.S. Pat. No. 6,073,050, describes a low-power Class-E transmitter as one type of the High-Q resonant circuit transmitter, operating at 49 MHz The '050 patent describes amplitude modulation of the transmitter to provide adjustable power to an implanted device, with no provision for FSK modulation of the Class-E converter. The operation of a Class-E converter was also described in prior art by Co-inventor Troyk (et al.) in U.S. Pat. No. 5,179,511 Self-regulating Class E Resonant Power Converter Maintaining Operation in a Minimal Loss Region. It is another object of this invention to offer a novel way to frequency modulate the converter previously described by Troyk, et al.
For the receiver circuitry contained within the internal (implanted) miniature device, in an application such as the one herein, the demodulation method chosen must provide a reliable data link, which also should not be compromised by variations in magnetic field strength. Such variations are typically due to movement of the internal electronics, or magnetic link, with respect to the external transmitter, movement of nearby metallic objects, or variations in the external transmitter power supply. In addition to these conditions, variations in power consumption of the implanted electronics will vary the nature of the current in the implanted coil. The data demodulation process must be immune to these variations or data errors may occur. Since the receiver is often to be implanted, or embedded, complete integration of the receiver electronics is required primarily for miniaturization, and secondarily for power minimization.
Several references related to data telemetry describe in detail conventional FSK demodulation techniques as a component of a proposed system (Kruse et al. U.S. Pat. No. 6,201,993, Hogrefe et al. U.S. Pat. No. 4,561,443). Others simply state that their embodiments may employ FSK or FM techniques to realize data transfer without offering specifics on how the FSK demodulation is to be performed (see, for example, Silvian U.S. Pat. No. 4,681,111, Urbas et al. U.S. Pat. No. 5,532,686, Juengel U.S. Pat. No. 4,742,470). In the '111 patent, the problem of response time for FSK demodulation is described. Traditionally, FSK demodulators suffer from the problem of slow response due to the need for either a phase-locked-loop, or other time-constant driven circuit to respond to changes in the FSK modulation. The slow response time limits the data rate of the system relative to the transmitter frequency.
Prior art also describes an FSK demodulator suitable for integration (Mukiayama U.S. Pat. No. 4,517,519) that compares zero crossing times of an input signal of unknown frequency with a known internal reference frequency. The '519 invention is significant in that it teaches a system that can be fully integrated on a single silicon substrate, an important requirement for miniaturized implantable devices. This technique is not applicable in certain important applications, because for these the implanted receiver has only the transmitter magnetic field as its clock, and no reference frequency is available. Therefore the '519 techniques cannot be used. It is a still further object of this invention to provide a transmitter/receiver system that overcomes these obstacles related to the availability of reference frequencies.
Mimura et al. U.S. Pat. No. 5,309,113, describe the use of a fixed delay and mixers to discern the input frequency. Hendrickson et al. U.S. Pat. No. 5,933,455, describe an FSK data slicer using the charging time of a capacitor as a time reference and over-sampling of the results to improve data reliability. It is a still further object of this invention to disclose a new approach to the problem that the unavailability of a time reference and over-sampling procedures require.
The invention described herein uses the charging time of multiple capacitors in a multi-phase fashion as a time reference. This eliminates the need for over-sampling and the need for a higher frequency clock. This is a particularly salutary feature since a frequency clock is not typically available in a miniaturized electronics application such as the one presented here.
The prior art does not disclose an FSK demodulation technique that automatically accommodates for asymmetries in the incoming waveform's detected period due to rectification artifacts. These rectification artifacts are characteristic of all transponder or medical implant implementations where power and data are transferred over a magnetic link simultaneously via a magnetic field. It is a still further object of this invention to addresses a significant need for a transmitter/receiver system that can operate without the need for a fixed time clock or over-sampling of data.